The image to the right shows an Untitled STI Sonic Game which was sort of a stepping stone in the development of Sonic X-treme. The image was created by Chris Senn whom was an employee of STI at the time. No actual game engine was created or any actual work was done to create a game based on the image. The concept art shows what could have been another game with a isometric view similar to that of SegaSonic the Hedgehog, Sonic 3D Blast, and Sonic Labyrinth. If a game were developed it may have been unlike Sonic 3D and featured a less wide open environment due to there being single column walls and platforms. It may have had more of a focus on platforming rather than exploring. From the single screen shown it's impossible to determine if this is entirely true, or if there was to be more open areas to move in.

According to Chris Senn this mockup was created before Sonic 3D was released and was during the development of the 32X.[1]